Cefdinir is used to treat a wide variety of bacterial infections. This medication is known as a cephalosporin antibiotic. It works by stopping the growth of bacteria. This antibiotic treats only bacterial infections. Side Effects Diarrhea, headache, or nausea may occur. If any of these effects persist or worsen, notify your doctor or pharmacist promptly. This medication may cause your stools to turn a reddish color, especially if you also take iron products, this is harmless side effect.